The present invention relates to a composition suitable for surface treatment of paper, in particular a size press liquor, and a process for whitening paper using said composition.
In the production of paper, a sizing step is usually carried out for achieving good writing and printing properties and strength. Such a sizing step can take place, on the one hand, before the sheet formation in the paper pulp (internal sizing) and, on the other hand, after the sheet formation in the size press. A combination of both processes is also possible. In one or both production stages of paper, whitening of the pulp or of the paper sheet is usually also carried out by means of a fluorescent whitening agent (FWA). Usually, the size and fluorescent whitening agent are added separately to the paper pulp in the case of pulp application, whereas the fluorescent whitening agent is incorporated into the size press liquor and applied together with it to the paper sheet in the case of surface sizing.
The combination of surface sizing and whitening of papers is widely used in the paper-producing industry. This method is widely used particularly in the printing and writing paper segment (copy, inkjet, offset, etc.). There is a continuing trend towards surface-sized papers having high whiteness and improved printing performance and consequently there is a demand for size press liquors which are as effective as possible. U.S. Pat. No. 6,207,258 B1 discloses a composition and process for improved inkjet printing performance using a salt of a bivalent metal, in particular calcium chloride. Therefore, in order to achieve more brilliant and sharper printings, especially inkjet printings, the paper production industry uses nowadays calcium chloride in size press liquors. However, the use of that salt affects adversely the performance of the fluorescent whitening agents commonly used in size press liquors. In particular, the whitening effectiveness is decreased, the shade is moving to the greenish-yellowish direction, and additionally a loss of fluorescence is observed.